In an essay in the November issue of O magazine, on newsstands today, Klebold describes the shock and horror she experienced on the day of the shooting, as well as her personal journey to comprehend her son's actions and move forward.

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On April 20, 1999, 17-year-old Dylan Klebold and fellow student Eric Harris, who was 18, opened fire inside their high school, killing 13 and injuring more than 20 before turning their weapons on themselves.

"For the rest of my life, I will be haunted by the horror and anguish Dylan caused," Klebold wrote. "I cannot look at a child in a grocery store or on the street without thinking about how my son's schoolmates spent the last moments of their lives. Dylan changed everything I believed about myself, about God, about family and about love."

O magazine editor at large Gayle King told "Good Morning America" today that it wasn't the anniversary of the shooting that inspired Dylan's mother to share her story, but instead, "she said it's really taken 10 years for her to process."

"She still feels unspeakable grief," King said. "As a parent, you think, how could you not know?"

Klebold was at work when she got a phone call from her husband telling her that there had been a shooting at the high school and Dylan was unaccounted for.

In her essay, Klebold describes an exchange she had with Dylan on the morning of the massacre and says she is haunted by not having seen him face-to-face that day.

"Early on April 20, I was getting dressed for work when I heard Dylan bound down the stairs and open the front door," she wrote. "Wondering why he was in such a hurry when he could have slept another 20 minutes, I poked my head out of the bedroom. 'Dyl?' All he said was 'Bye.' The front door slammed, and his car sped down the driveway.

"His voice had sounded sharp. I figured he was mad because he'd had to get up early to give someone a lift to class. I had no idea that I had just heard his voice for the last time."

King said Klebold "replays that scene a million times in her head."

Dylan Klebold's Journals Broke Mother's Heart

In the decade since the Columbine massacre, the nation has learned that Dylan, who was in a program for highly intellectual students and was his father's loyal chess partner, had grown increasingly shy in his teenage years. He was bullied and turned inward to journals that describe a depressed teen who hated life.

"I'm an outcast," he wrote in one entry, "and everyone is conspiring against me."

He also wrote about events like the Oklahoma City bombing, and wanting to outdo those events.

"From the writings Dylan left behind, criminal psychologists have concluded that he was depressed and suicidal," Susan Klebold wrote in her essay. "When I first saw copied pages of these writings, they broke my heart. I'd had no inkling of the battle Dylan was waging in his mind."

King said Klebold "believes that her son had not been diagnosed with depression, that he had a mental illness that she was not aware of. She also doesn't believe that if he wasn't suicidal, that these murders would have happened."

Klebold described Dylan as a joyful and intellectually adventurous child who changed dramatically after he entered junior high school, becoming quiet, unmotivated academically and irritable.

Despite that, his mother says he had been accepted to four colleges and was looking forward to attending the University of Arizona, and had attended his high school prom.

It was only in the days, weeks and months after her son's death that she learned the extent of his depression and his involvement in planning the shooting. Klebold and her husband had no idea that Dylan and Harris had been compiling a cache of weapons, and she had never seen the journal entries in which Dylan wrote frequently about suicide.

"Dylan's participation in the massacre was impossible for me to accept until I began to connect it to his own death," she wrote. "Once I saw his journals, it was clear to me that Dylan entered the school with the intention of dying there. And so in order to understand what he might have been thinking, I started to learn all I could about suicide."

She wrote that she never knew about his dark side.

"Yes, he had filled notebook pages with his private thoughts and feelings ... but we'd never seen those notebooks," she wrote. "And yes, he's written a school paper about a man in a black trench coat who brutally murders nine students. But we'd never seen that paper."

King said, "On the day when she went for the parent-teacher conference, they didn't have the paper there."

Mixed Reactions From Columbine Victims' Families

"I can't help but think that a lot of this was preventable," Brad Bernall said.

Susan Klebold does partially blame herself.

"I often wished that I would die," she wrote. "While I perceived myself to be a victim of the tragedy, I didn't have the comfort of being perceived that way by most of the community. I was widely viewed as a perpetrator or at least as an accomplice since I was the person who had raised a 'monster.'"

She sent some of the victims and their families apology letters, but stopped sending them on her therapist's advice because some families found them unsettling.

"We've never received a personal apology from the Harrises or Klebolds," Misty Bernall said. "In those early days, it would've meant a lot."

But Brian Rohrbough, who lost his 15-year-old son Daniel, said he's relieved to finally hear from Susan Klebold.

"I'm happy to see it now," he said. "It appears to me that she is saying that she is doing this in hopes of preventing another suicide or another murder-suicide."

Klebold has struggled both with her grief and feelings of guilt about the shooting. Still, the family hasn't left the community and she now works to encourage suicide research, prevention and survivor support. Klebold hopes this will help other parents recognize the signs that she didn't.

"We should ask ourselves how good children are at hiding things," King said. "She said, 'we did not have those kind of clues.'"

Klebold wrote, "I concluded that he must not have loved me, because love would have prevented him from doing what he did. I'd failed to see that he needed help."

For the full essay by Susan Klebold, pick up the November issue of O magazine and visit oprah.com/essay